The storage device includes a volatile first memory circuit and a nonvolatile second memory circuit which includes a transistor whose channel is formed in an oxide semiconductor layer. In the case of high-frequency driving, during a period when source voltage is applied, a data signal is input to and output from the first memory circuit, and during a part of a period when source voltage is supplied, which is before the supply of the source voltage is stopped, a data signal is input to the second memory circuit. In the case of low-frequency driving, during a period when source voltage is applied, a data signal is input to the second memory circuit, the data signal input to the second memory circuit is input to the first memory circuit, and the data signal input to the first memory circuit is output.